Viscoelastic surfactant fluids are normally made by mixing in appropriate amounts suitable surfactants such as anionic, cationic, nonionic and zwitterionic surfactants. The viscosity of viscoelastic surfactant fluids is attributed to the three dimensional structure formed by the components in the fluids. When the concentration of surfactants in a viscoelastic fluid significantly exceeds a critical concentration, and in most cases in the presence of an electrolyte, surfactant molecules aggregate into species such as micelles, which can interact to form a network exhibiting elastic behavior. In the remaining part of this description, the term “micelle” will be used as a generic term for the organized interacting species.
Viscoelastic surfactant solutions are usually formed by the addition of certain reagents to concentrated solutions of surfactants, frequently consisting of long-chain quaternary ammonium salts such as cetyltrimethylammonium bromide (CTAB). Common reagents that generate viscoelasticity in the surfactant solutions are salts such as ammonium chloride, potassium chloride, sodium salicylate and sodium isocyanate and non-ionic organic molecules such as chloroform. The electrolyte content of surfactant solutions is also an important control on their viscoelastic behaviour.
There has been considerable interest in using such viscoelastic surfactants as wellbore service fluids. Reference is made for example to U.S. Pat. No. 4,695,389, No. 4,725,372, No. 5,551,516, No. 5,964,295, and No. 5,979,557.
Introduction of additional components to the fluid can cause a dramatic decrease in the fluid viscosity, called “breaking”. This can occur even with components, such as water or electrolytes, that may already be present in the fluid. For example, in oilfield applications, the viscosity of viscoelastic surfactant fluids is reduced or lost upon exposure to formation fluids (e.g., crude oil, condensate and/or water); and this viscosity reduction or loss effectuates cleanup of the reservoir, fracture, or other treated area.
However, in some circumstances, it would be suitable to have a better control of that breaking, for instance, when breaking of the fluid is desired at a particular time or condition, when it is desired to accelerate viscosity reduction, or when the natural influx of reservoir fluids (for example, in dry gas reservoirs) does not break or breaks incompletely the viscoelastic surfactant fluid. This disclosure describes compositions and methods employed to break viscoelastic surfactant fluids.
Gel breakers are of common use for conventional polymer based fluids used in stimulation and the like since, unlike viscoelastic surfactant based fluid, conventional based polymers fluids do not spontaneously break when contacted by hydrocarbons or aqueous formation fluids and leaving a high-viscosity fluid in the formation would result in a reduction of the formation permeability and consequently, a decrease of the production. The most widely used breakers are oxidizers and enzymes. The breakers can be dissolved or suspended in the liquid (aqueous, non-aqueous or emulsion) phase of the treating fluid and exposed to the polymer throughout the treatment (added “internally”), or exposed to the fluid at some time after the treatment (added “externally”). The most common internal methods and compositions for conventional polymer based systems involve soluble oxidizers or enzymes; the most common external methods and compositions involve encapsulated enzymes or encapsulated oxidizers or involve the use of pre- or post-flushes that contain breakers. Breaking can occur in the wellbore, gravel-pack, filter cake, the rock matrix, in a fracture, or in another added or created environment.
UK Patent GB2332223, “Viscoelastic surfactant based gelling composition for wellbore service fluids” by Hughes, Jones and Tustin describes methods to delay and control the build-up of viscosity and gelation of viscoelastic surfactant based gelling compositions. These methods are used to facilitate placement of the delayed (“pre-gel”) fluid into a porous medium and then to trigger formation of the viscoelastic gel in-situ.
Rose et. al. describe in U.S. Pat. No. 4,735,731 several methods to reversibly break the viscosity of VES solutions through an intervention at surface. These methods include heating/cooling the fluid, adjusting the pH or contacting the fluid with an effective amount of a miscible or immiscible hydrocarbon and then, subjecting the fluid to conditions such that the viscosity of the fluid is substantially restored. The reversible treatment of Rose is useful for drilling fluids so that the fluid pumped into the well is viscous enough to carry cuttings to the surface but able to be broken at surface for solids removal. The breaking methods discussed in Rose are not used to break a viscoelastic solution down a well and further appear to have an immediate impact on the viscosity of the fluid.
Therefore, there exists a need for methods for breaking viscoelastic surfactant fluids after subterranean oil or gas well treatments, at predetermined times or conditions and/or when they are not broken by the natural influx of reservoir fluids.